1. Field of the Invention
The present invention relates to a method for forming fine patterns of a semiconductor device, whereby the high integration of a semiconductor device can be achieved.
2. Description of the Prior Art
Certainly, the recent tendency of high integration is greatly dependent on the developmental progress of techniques for forming fine patterns. For example, photoresist patterns are in general used as masks in etching or ion implantation and their fineness is, therefore, a very important factor in determining the degree of integration.
In this regard, a description will be given of conventional methods for forming fine photoresist patterns.
In an example, a semiconductor substrate on which a predetermined infrastructure has been established and curved patterns will be formed, is coated with a photoresist solution made by dissolving a certain ratio of photoresist agent and resin in a solvent, to give a photoresist film.
Subsequently, an exposure mask is prepared on a transparent substrate at the position which corresponds to the pattern to be formed in the photoresist film.
Next, light is illuminated through the exposure mask on the photoresist film to selectively polymerize the predetermined part of the photoresist film.
Then, the resulting semiconductor substrate is subjected to soft bake at 80-120.degree. C. for 60-120 sec, followed by the development of the semiconductor substrate. For the development, a weak alkali solution consisting mainly of tetramethylammonium hydroxide is used to selectively remove the exposed/unexposed regions of the photoresist film.
Finally, the semiconductor substrate is washed with deionized water and dried, to obtain a photoresist pattern.
The resolution (R) of the photoresist pattern is proportional to the wavelength (.lambda.) of the light source from a stepper and to the process parameter (k) and inversely proportional to the numerical aperture of the steper. Thus, in order to enhance the optical resolution of the stepper, a light source with short wavelength may be employed. For example, G-line (436 nm) and i-line (365 nm) steppers show limited process resolutions of about 0.7 and 0.5 .mu.m, respectively. Shorter wavelength, for example, deep ultraviolet, is recruited for finer patterns with a size smaller than 0.5 .mu.m. For example, the exposure process is carried out in a steper using as a light source a KrF laser with a wavelength of 248 nm or an ArF laser with a wavelength of 193 nm. Also, a contrast enhancement layer (CEL) or a phase shift mask may be used.
However, there is a limit in the recruitment of short wavelength as a light source and the CEL process is complicated by a poor production yield.
In order to better understand the background of the invention, another conventional method for forming fine patterns of semiconductor device will be described in conjunction with FIG. 1.
First, as shown in FIG. 1, a layer 2 to be etched, a lower photoresist film 3, an intermediate layer 4 and an upper photoresist film 5 are formed in sequence on a semiconductor substrate 1.
Next, an exposure mask 10 is prepared by forming narrow spaced light screen patterns 8 on a transparent substrate 6.
Then, light is illuminated through the exposure mask 10 selectively on the entire semiconductor substrate to form fine patterns (not shown).
According to another conventional technique, fine patterns are obtained by exposing a conventional tri-layer resist (hereinafter referred to as "TLR") two times to a light source via two exposure masks whose patterns are spaced enough not to generate the diffraction of the illuminated light.
In this TLR process, the process parameter is so small that the resolution is enhanced by more than 30% that obtainable in a mono-layer resist process. Also, the two times exposure in the TLR process allows the patterns to be finer than those obtained by using an ordinary mask.
However, the patterns less than 0.25 .mu.m in size, necessary for highly integrated semiconductor devices of 256 M or higher, are difficult to obtain with the TLR process. In addition,.since the previously formed photoresist patterns remain at a thickness of 0.4 .mu.m during the repeat of photoresist coating and exposure, the photoresist film coated in the second round is poor in uniformity, which causes the depth of focus to become small and generates problems, such as CD biasing or notching, resulting in degradation in production yield and operation.
Therefore, it is apparent that all the above conventional techniques are not suitable for the high integration of a semiconductor device.